An organic EL device is a self light-emitting device, and is thus brighter, better in visibility, and capable of clearer display, than a liquid crystal device. Hence, active researches have been conducted on organic EL devices.
In 1987, C. W. Tang et al. of Eastman Kodak Company developed a laminated structure device sharing various roles for light emission among different materials, thereby imparting practical applicability to organic EL devices. The developed organic EL device is configured by laminating a layer of a fluorescent body capable of transporting electrons, and a layer of an organic substance capable of transporting holes. As a result of injecting positive charges and negative charges into the layer of the fluorescent body to perform light emission, it is possible to obtain a high luminance of 1000 cd/m2 or higher at a voltage of 10 V or less.
Many improvements have been heretofore made to put the organic EL devices to practical use. It is generally well known that high efficiency and durability can be achieved by an electroluminescence device having a laminated structure, in which the roles to be played by respective layers are further segmented, i.e., having an anode, a hole injection layer, a hole transport layer, a luminous layer, an electron transport layer, an electron injection layer, and a cathode on a substrate.
For further increase in the luminous efficiency, it has been attempted to utilize triplet excitons, and the utilization of phosphorescent luminous compounds has been investigated.
Furthermore, devices utilizing light emission by thermally activated delayed fluorescence (TADF) have been developed. For example, in 2011, Adachi et al. from Kyushu University have realized an external quantum efficiency of 5.3% with a device using a thermally activated delayed fluorescence material.
The luminous layer can also be prepared by doping a charge transport compound, generally called a host material, with a fluorescent compound, a phosphorescent luminous compound, or a material radiating delayed fluorescence. The selection of the organic material in the organic EL device greatly affects the characteristics of the device, such as efficiency and durability.
With the organic EL device, the charges injected from both electrodes recombine in the luminous layer, thereby producing light emission, and how efficiently the charges of the holes and the electrons are passed on to the luminous layer is of importance, and a device that exhibits excellent carrier balance is required. Further, by enhancing hole injection property or increasing electron blocking property, that is, property to block electrons injected from the cathode, it is possible to increase the probability of holes and electrons recombining. Besides, excitons generated in the luminous layer are confined. By so doing, it is possible to obtain a high luminous efficiency. Therefore, the role of the hole transport material is important, and a demand has been created for a hole transport material having high hole injection property, high hole mobility, high electron blocking property, and high durability to electrons.
Further, from the viewpoint of device life, heat resistance and amorphousness of the materials are also important. A material with a low heat resistance is thermally decomposed even at a low temperature by heat produced during device driving, and the material deteriorates. In a material with low amorphousness, crystallization of a thin film occurs even in a short time, and the device deteriorates. Thus, high heat resistance and satisfactory amorphousness are required of the materials to be used.
N,N′-diphenyl-N,N′-di(α-naphthyl)benzidine (NPD) and various aromatic amine derivatives are known as hole transport materials which have been heretofore used in organic EL devices (see, for example, PTL 1 and PTL 2). NPD has satisfactory hole transport capability, but the glass transition temperature (Tg), which is an indicator of heat resistance, is as low as 96° C. and device characteristics degrade due to crystallization under high-temperature conditions.
Further, among the aromatic amine derivatives, there are also compounds with an excellent hole mobility of 10−3 cm2/Vs or higher (see, for example, PTL 1 and PTL 2). Since electron blocking property is insufficient, however, some of electrons pass through the luminous layer, and no increase in luminous efficiency can be expected. Thus, materials with better electron blocking property, higher stability of a thin film, and high heat resistance are needed to increase further the efficiency.
An aromatic amine derivative with high durability has also been reported (see, for example, PTL 3), but this derivative is used as a charge transport material for use in an electrophotographic photosensitive body and there is no example of application to an organic EL device.
Arylamine compounds having a substituted carbazole structure have been suggested as compounds with improved properties such as heat resistance and hole injection property (see, for example, PTL 4 and PTL 5). Although heat resistance, luminous efficiency, and the like of devices using these compounds for a hole injection layer or hole transport layer have been improved, the results are still insufficient and further decrease in a driving voltage and increase in luminous efficiency are needed.
Devices in which holes and electrons can recombine with a high efficiency and which have a high luminous efficiency, a low driving voltage, and a long life need to be provided by combining materials with excellent hole and electron injection-transport performance and stability and durability of a thin film so as to improve device characteristics of organic EL devices and increase the yield in device production.
Further, devices which have carrier balance, a high efficiency, a low driving voltage, and a long life need to be provided by combining materials with excellent hole and electron injection-transport performance and stability and durability of a thin film so as to improve device characteristics of organic EL devices.